In the electric utility industry, plug-in, socket-type, watt-hour meters are commonly used to measure electric power consumption at residential or commercial sites. The most common type is more properly known as a kilowatt hour meter or a joule meter. When used in electricity retailing, the utilities record the values measured by these meters to generate an invoice for the electricity. These meters may also record other variables including the time when the electricity was used.
The socket for the watt-hour meter is usually installed in a housing that is mounted on a wall of the residence or commercial building. Typically, the housing is transparent or has a window so that the meter can be read without opening the housing. The meter socket contains line and load terminals which are respectively connected to electric line and load connectors. The line and load connectors are connected to cables providing electrical power to/from the meter socket. The terminals receive the blade contacts of a plug-in watt-hour meter to complete an electric circuit through the meter between the line and load terminals.